1. Field of the Invention
The present invention relates generally to a system and method for analyzing data and more particularly for analyzing data gathered by an atomic force microscope and determining characteristics of a sample.
2. Related Art
Atomic force microscopy (AFM) is rapidly emerging as an important tool in microrheology and nanotechnology. The ability of AFM to create three dimensional micrographs with resolution down to the nanometer and angstrom scales has made it an essential tool for imaging surfaces in applications ranging from semiconductor processing to cell biology. The AFM can also probe nanomechanical and other fundamental properties of sample surfaces including their elastic and adhesive properties.
An example of a typical AFM is shown in FIG. 1. The AFM 10 includes a cantilever 12 having a tip of a known shape 14 that is used to sense a force between the sample 16 and the tip 14. As the tip 14 is moved along the surface of or perpendicular to the sample, the tip 14 deflects depending upon the force exerted on the tip 14. An electromagnetic beam 18, such as a laser beam, is directed at a reflective surface on the end of the cantilever 12. The beam 18 is reflected from the cantilever. The reflected beam 20 is detected by a photodetector 22, such as a split photodiode. Movement of the tip can be correlated to the movement of the reflected beam 20 on the photodetector 22. The size and position of the current created in the photodetector 22 are linked via detector electronics 24 to a control computer 26. A feedback loop between the detector electronics 24 and the control computer 26 is provided to maintain the cantilever position at a defined location on the surface that is being analyzed. The control computer 26 controls the movement of the microscope along the X, Y and Z-axes. Fine motion piezoelectric controllers are used to generate the precise motion that is needed to generate topographic images and force measurements. A piezoelectric controller is a device that moves by a precise amount when a voltage is applied across its electrodes. The piezoelectric controllers are used to control and place the tip 14 along the surface of the sample 16. The nanoscale deflections of the tip and the feedback voltages required to maintain this deflection can provide many different types of data regarding the sample. For example, reliable data relating normal forces to indentation depths of the sample can be calculated. This data can be used to determine the modulus of elasticity for the sample. In one specific example, the modulus of elasticity can be detected for small tissue samples or even on individual cells. This affords new and potentially invaluable information regarding tissue and cell properties and can lead to improvements in predictive and heuristic models of the behavior of biomaterials.
The force sensed by the AFM is calculated by multiplying the deflection of the cantilever by the cantilever's spring constant. This force is then typically plotted versus cantilever position as a force curve. The force curve provides a graphic way of seeing how much force is exerted on a cantilever probe at a given cantilever position. However, the quantitative analysis of the cantilever position and tip deflection data is extremely difficult and time-consuming. Few people approach the problem in the same way. Additionally, there is considerable debate concerning the best method of dealing with nonlinearities inherent in AFM, which portion of the force curve to analyze, or whether the data should be evaluated at constant force or at constant indentation. These problems are compounded by the uncertainty in determining an initial point of contact of the tip with the sample. Thus, a great deal of subjectivity is introduced into standard AFM analyses.
The use of idealized models of contact mechanics to describe real data may require solving an assortment of technical problems. Foremost among these in this application is the difficulty of determining the exact point at which the tip contacts the sample. Known contact forces including attractive electrostatic charges and repulsive coulombic forces can induce tip motion and complicate the assessment of tip contact. Additionally, certain models used in determining the force and elasticity of the sample require certain assumptions, which are not always appropriate.
Thus, there is a need for a method and system which can minimize errors in interpreting AFM data. The system and method should allow for a systematic diminution of the potential error in AFM analyses. The system should be automated and be able to process large amounts of data accurately in a short period of time.